Field
The invention relates to an anchoring member suitable for use in a polyaxial bone anchoring device and to a polyaxial bone anchoring device with a pivot angle that is enlarged to one side. The anchoring member comprises a bone anchoring section and a head with a common central axis and a neck portion between the bone anchoring section and the head, wherein the neck portion is asymmetric with respect to the central axis. The asymmetry of the neck portion is established by a recess that is present at one side of the neck portion and an extension or projecting portion that is present at the opposite side of the neck portion. A polyaxial bone anchoring device including such an anchoring member permits pivoting of the anchoring member to at least one side with an enlarged pivot angle.
Description of Related Art
A bone anchoring device with an enlarged pivot angle to one side is described in U.S. Pat. No. 6,736,820. The bone anchoring device comprises a bone screw and a receiving part with an open first bore and a substantially U-shaped cross-section for receiving the rod and a second bore on the end opposite to the first bore and a seat for the head. In one embodiment, the screw member has a neck between a head and a threaded section, the neck having a recess or countersink and being asymmetric with respect to the symmetry axis. As a result thereof, the screw member can pivot at a larger angle to the side where the recess or countersink is present.
Another bone anchoring device with increased angulation is known from U.S. Pat. No. 8,029,539 B2. It includes a screw having a head, a shank and an intermediate or neck portion. The neck portion comprises a first surface or area having a relatively large radius and a second surface or area that has a relatively small radius. The larger and smaller radiuses of the surfaces permit the retainer of the anchoring device to be pivoted at a plurality of different angles or degrees.